This invention relates to the field of variable resistors. More particularly, it relates to variable resistors adapted for being surface mounted on a circuit board.
Surface mount variable resistors and potentiometers are well known in the art. Such devices typically are very small (miniature or subminiature, i.e., less than about 6 mm on a side), single turn devices, employing an ink film (e.g., cermet) resistive element on an insulative substrate. The resistive element is contacted by a wiper element that is mounted on the underside of a rotor that is rotatably attached to the substrate In many such devices, the wiper element is formed from a plate or slider that is disposed on the underside of the rotor, spaced from, and opposed to, the surface of the substrate on which the resistive element is carried The wiper is in the form of a finger or arm that is stamped from the slider, and bent out of the plane of the slider so as to be resiliently engageable against the resistive element Several prior art variable resistors of this general type are disclosed in U.S. Pat. No 4,821,014--Masuda et al., and in published Japanese Patent Applications Nos. 62-193025 and 62-225850.
FIG. 1 accompanying this specification illustrates a simplified version of a rotor/slider assembly 10, in accordance with the teachings of the above-referenced patent to Masuda et al. In this prior art device, a metallic slider 12, having an integral wiper arm 14 stamped therefrom, is insert-molded into a rotor 16, formed from a heat-resistant, thermoplastic resin. The slider is formed from a single metal plate having a pair of spaced-apart apertures. The plate is folded over itself with the apertures in mutual registration, forming a double-layer, substantially annular slider base, with a first, or inner, layer 12a lying flush against the adjoining surface of the rotor 16, and a second, or outer, layer 12b that partially covers the inner layer. The outer layer 12b has a circumferential gap defined between a pair of radial edges 12c, through which gap a portion of the inner layer 12a is exposed. The wiper arm 14 extends over the gap from one of the edges 12c, and is stamped out of the portion of the plate forming the outer layer 12b, leaving the aforementioned gap.
The use of the double layer design for the slider 12 is necessitated by the gap formed when the wiper arm 14 is stamped out of the plate from which the slider is formed. Because of this gap, a backing plate is needed during the insert molding process to shut off the mold in the area of the wiper. This backing plate function is provided by the inner layer 12a of the slider. Thus, one layer (the inner layer 12a) is needed as a backing plate or mold shut-off plate, while the other layer (the outer layer 12b) is needed as the wiper carrier,
There are several disadvantages to the above-described arrangement. First, with an integral wiper/slider assembly, the wiper must necessarily be of the same material as the rest of the slider, thereby affording a narrower range of spring rates for the wiper than would be available by the use of different material. Second, the need to fold the plate that forms the slider, and the need to do so with sufficient precision to align the apertures, adds complexity to the fabrication process. In addition, a potential leak path is provided through the central rotor aperture (indicated by the numeral 18 in FIG. 1), between the rotor 16 and a central post (not shown) that extends through the central aperture 18, and around which the rotor 16 rotates.
Therefore, there has been a need for a surface mount variable resistor that allows for simpler construction, better sealing against leaks, and a broader range of wiper spring rates than has heretofore been possible in such devices, while maintaining the compactness of size and relative simplicity that have characterized the prior art.